Reducing mechanical loss in an internal combustion engine such as a vehicular engine is effective to improve fuel consumption. Thus, attempts to reduce frictional losses are being developed with respect to various components.
For example, friction loss in the movement range between a piston and a cylinder bore in an internal combustion engine is considered to account for 20 to 30% of mechanical loss in the entire internal combustion engine. In order to reduce friction especially between a piston ring and a cylinder bore, various suggestions to reduce piston ring tension have been proposed.
Piston ring tension has a close relationship with a sealing property in a combustion chamber, and has an influence on engine oil consumption. In view of this, in addition to simply reducing piston ring tension, a means of providing a surface treatment on a piston ring having a small fluctuation of piston ring tension having a rigid thin coating such as CrN and excellent abrasion resistance, and a means of reducing surface roughness of a piston ring (refer to Patent Document 1) have been proposed.